1. Field of the Invention
The present invention relates to an automotive air conditioning system for heating the interior of the passenger compartment of a vehicle, and more particularly to an automotive air conditioning system with a hot gas cycle for heating air passing through an air conditioning duct with an interior heat exchanger (an evaporator) provided inside the passenger compartment by causing hot, high-pressure gas refrigerant discharged from a refrigerant compressor to bypass an exterior heat exchanger (a condenser) provided outside the passenger compartment so as to be introduced directly to a pressure reducing device and then to the interior heat exchanger in that order.
2. Description of the Related Art
Generally used as an automotive air conditioning system or, in particular, as an automotive heating system, is a hot-water heating system for heating the interior of the passenger compartment of a vehicle by introducing engine cooling water to a hot-water heater located in an air conditioning duct so as to heat air passing through the air conditioning duct. With the hot-water heating system of this type, however, there is caused a drawback that a remarkable shortage of heating capacity is experienced, just after the hot-water heating system has been started, in the case where the heating system is activated, by starting the engine, when the temperatures of the outside air and cooling water are low.
With a view to eliminating the drawback, there has been proposed an automotive air conditioning system with a hot gas cycle which functions as an auxiliary heating system designed to assist the heating work of a main heating system, for heating the vehicle interior by making use of engine cooling water, by causing hot, high-pressure gas refrigerant (hot gas) compressed and discharged by a refrigerant compressor in a normal refrigeration cycle to bypass an exterior heat exchanger (a refrigerant condenser) so that the gas refrigerant is passed through a pressure reducing device and is then supplied to an interior heat exchanger (a refrigerant evaporator) located within an air conditioning duct so as to heat air passing through the air conditioning duct. Note that the refrigerant compressor is belt driven by the engine via an electromagnetic clutch.
When attempting to heat the vehicle interior quickly, in the event that the temperature of the engine cooling water is equal to or higher than a predetermined temperature, as the heating capacity of the main heating system is good enough to heat the vehicle interior, the refrigerant compressor is controlled to be switched off so as to stop the hot gas cycle, while in the event that the temperature of the cooling water is lower than the predetermined temperature, as the heating capacity of the main heating system is not sufficient to do the required work, the refrigerant compressor is controlled to be switched on so as to activate the hot gas cycle.
Here, the hot gas cycle works differently from a normal heat pump cycle in which the interior heat exchanger provided inside the passenger compartment is caused to work as a refrigerant condenser and the exterior heat exchanger provided outside the passenger compartment is caused to work as the refrigerant evaporator and is designed such that heat resulting from the compression work of the refrigerant compressor is dissipated from the interior heat exchanger (the refrigerant evaporator in the cooling mode), and therefore, the hot gas cycle can operate at extremely low outside air temperatures such as those in the vicinity of −40° C.
In the related art, however, when the temperature is extremely low and is, for example, −20° C. or lower, due to the property of the refrigerant called HFC-134a, there occurs a case where a negative pressure (a pressure lower than the atmospheric pressure, such as an atmospheric pressure of 0 kg/cm2G at −26° C.) is created before the refrigerant compressor is activated. In addition, due to the property of the hot gas cycle, as an intake pressure taken into an intake portion of the refrigerant compressor just after the compressor has been activated tends to rise gradually after the intake pressure has lowered once, it is confirmed that the frequency at which the negative pressure is created at the intake portion of the refrigerant compressor is very high.
Incidentally, as to the fact that the negative pressure is created at the intake portion, in engine-driven type refrigerant compressors that are currently used commonly, a shaft seal portion is found to be weak, and when the intake pressure of the refrigerant compressor lowers to or below a predetermined value (for example, −0.5 kg/cm2G), there is caused a possibility that air enters the interior of the refrigerant compressor from the shaft seal portion. As an ingress of air into the interior of the refrigerant compressor like this results in the generation of corrosion within the cycle or an abnormal rise of the high pressure (condensing pressure) at the time of cooling, such an ingress of air must be prevented.
With a view to eliminating the above drawback, in a related art disclosed in U.S. Pat. No. 6,237,681, in the event that a physical amount related to an intake pressure taken into a refrigerant compressor, that is detected by an intake pressure detecting device, becomes smaller than a predetermined value when a hot gas cycle is activated in a condition where the outside air temperature is extremely low, the refrigerant compressor is automatically stopped so as to stop the operation of an auxiliary heating system, whereby a reduction in intake pressure of the refrigerant compressor, that occurs just after the refrigerant compressor has been activated, can be prevented, the generation of a negative pressure at the intake portion of the refrigerant compressor being thereby prevented.
Furthermore, under a low temperature environment where the outside air temperature falls to or below −10° C., as the temperature and pressure of the refrigerant decrease to thereby increase the specific volume of the refrigerant, the weight flow rate of refrigerant, that is taken into the refrigerant compressor, becomes smaller and hence, the refrigerant compressor does not work sufficiently, resulting in a shortage of an auxiliary heating capacity.
With a view to eliminating the drawback, in a related art disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-42934, as shown in FIG. 8, a refrigerant heating unit 9 is provided for heating a refrigerant flowing through a refrigerant flow path 30 extending from an exit of a refrigerant evaporator 6 to an intake opening of a refrigerant compressor 7. The provision of the refrigerant heating unit 9 increases the temperature and pressure of refrigerant that is taken into the refrigerant compressor 7, and the specific volume of the refrigerant is decreased, while the weight flow rate of the refrigerant is increased, whereby the refrigerant compressor is allowed to work sufficiently, thereby making it possible to increase the auxiliary heating capacity.
In the related art disclosed in the above Japanese Unexamined Patent Publication, however, while the auxiliary heating capacity is increased by heating a low pressure side (from the exit of the refrigerant evaporator to the intake opening of the refrigerant compressor) of the hot gas cycle, the related art was not effective against a drawback that internal components of the refrigerant compressor wear or fail due to an ingress of air into the interior of the refrigerant compressor resulting from the generation of a negative pressure, which is lower than the atmospheric pressure, occurring in the interior of the compressor when the hot gas cycle is activated in a condition where the outside air temperature is extremely low, or insufficient lubrication of the interior of the refrigerant compressor resulting from no return of refrigerant and oil to the compressor after it has been activated due to the low pressure and density of the refrigerant.
However, the inventors of the present invention recognized that the temperature of refrigerant within the hot gas cycle (in particular, within the refrigerant compressor) tends to continue to increase at all times once the engine has been started. This tendency was considered to be grounded on the facts that heat generated from the engine is conducted to the refrigerant compressor, as the refrigerant compressor is fixed to the engine, and that heat building up inside the engine compartment is conducted to the interior of the hot gas cycle. The inventors then considered, based upon this rationale, that, as the weight of refrigerant per volume increases and the temperature within the hot gas cycle increases with passage of certain time without estimating an intake pressure, the generation of a large magnitude of negative pressure in the refrigerant compressor can be prevented and the return of refrigerant and oil from the refrigerant condenser and so on can be improved.